To use a parabolic satellite antenna, the antenna and its feed must be properly aligned with the satellite. Many satellite transmissions are polarized, and thus the antenna assembly must also be rotated to the proper polarity orientation. A simple parabolic antenna is configured such that the central axis of the parabola passes through the center of the dish. Once the antenna is aimed at the satellite, it may be rotated around this axis to adjust the polarity without losing the satellite alignment. As can be appreciated, however, because the parabolic axis is centrally located, the feed must be centrally positioned in front of the antenna. This configuration blocks a portion of the signal and results in a performance degradation.
To address this problem, many parabolic antennas are designed so that the parabolic vertex is near or at the antenna's lower edge. Antennas of this type are commonly referred to as offset parabolic antennas, since the feed is offset from the antenna edge, thus reducing the signal blockage. Such an antenna arrangement 10 is illustrated in FIG. 1. As shown, a parabolic antenna 12 has a vertex 14 which is adjacent its lower edge 16. A line 18 running between the vertex 14 and the focal point 20 defines a boresight to the satellite. A feed horn 22 is mounted at the focal point 20 and directed toward the dish 12 to receive reflected energy. The antenna dish 12 is mounted on an antenna mount 24. For reasons of balance, antenna mount 24 is generally attached to antenna 12 at or near the middle of the antenna.
The mount 24 is configured to allow the azimuth elevation of the antenna to be adjusted. In addition, the mount 24 is configured to allow the antenna to be rotated about a polarity axis 26 which is parallel to the boresight axis 18. The polarity axis is shifted from the perpendicular relative to the antenna attachment points. Thus, the mount 24 must be configured with this angular deviation in mind.
FIGS. 2a and 2b are side and back views, respectively, of a conventional mounting assembly 24. To compensate for the fact that the vertex of the antenna 12 is not centrally located, a separately mounted polarity plate 30 is provided. Polarity plate 30, generally formed using stamped steel, affixed to a predetermined location in the back of the antenna 12 after the antenna is manufactured, e.g., by means of bolts 31. As can be appreciated, both the boresight axis 18 and the polarity axis 26 are perpendicular to a polarity plane 28. The polarity plate 30 has a flat back plane 32 and is dimensioned such that when the plate 30 is bolted to the back of antenna 12 at the appropriate locations, the back plane 32 is perpendicular to the polarity axis 26. Rotating the antenna 12 about an axis perpendicular to the back plane 32 permits the polarity of the antenna to be adjusted without altering the directional satellite alignment.
The satellite alignment mechanisms includes a pair of elevation brackets 34 with locking bolts 36 that engage elevation slots 37 and are configured to allow adjustment of the elevation of the antenna. As shown in FIG. 2b, the polarity plate 30 include slots 38 formed in the back plane 32. The brackets 34 are attached to the polarity plate 30 via bolts 39 which engage slots 38 in the polarity plate 30 so as to allow the antenna to rotate about the polarity axis.
Although accepted in the industry, the use of the separately attached stamped polarity plate, such as shown in FIGS. 2a and 2b, has several drawbacks. First, the use of the additional components increases the cost of the antenna and assembly complexity. Separately formed components also decrease accuracy since fabrication tolerance errors for each component are added together, both in the angle of the back plane and the position at which the polarity plate is attached to the antenna. The separately attached plate also increases the number of mechanical stress points on the back of the antenna, and thus may reduce the performance and lifetime of the antenna under wind loads, particularly when the antenna is large. Additionally, care must be taken to ensure that when the antenna mount is assembled, the proper polarity plate 30 is attached. This is a particular concern when several antenna variations, perhaps having different dimensions and vertex locations are assembled in the same place or by the same personnel. If an improper back plane is attached, the error may not be noticed until after the antenna is fully assembled and installed, resulting in potentially costly repair.
Accordingly, it would be beneficial to provide an antenna arrangement which does not require the use of a separately mounted polarity plate.
It would also be advantageous to provide a molded antenna, such as a parabolic antenna, which includes integrally molded mounting components that are configured to provide the proper polarity plate without requiring the attachment of additional reference hardware.